Thanks for the fast response Ray. I normally use Fing to find network device MACs. Interestingly (to me) when it scans it sees the TP-Link device and it’s MAC address as assigned to the OpenSprinkler’s IP, and doesn’t show the OpenSprinkler MAC at all. My router also reports the same as Fing, so it is a bit misleading (not your issue!).
I changed the DHCP reservation to use the MAC from the LCD display and it seems to be working ok.